The analysis of complex mixtures such as biological fluids by gas chromatography often requires the use of selective detectors to discriminate against interfering species and/or enhance the response to desired compounds. However, with the exception of mass spectrometers, most GC detectors are either difficult or impossible to operate in a wide variety of selective and nonselective modes. Although not designed to provide the range and scope of data provided by mass spectrometers, the ion mobility spectrometer is capable of quantitative and low resolution qualitative detection of organic compounds. When compared with commonly employed selective detectors such as the electron capture and nitrogen-phosphorus detectors, the ion mobility spectrometer offers the distinct advantage of tunable selectivity. Because of its versatility and low cost when compared to that of mass spectrometers, the instrument has great potential for routine screening of many drugs in biological samples. During the past funding period we have expanded the capabilities of ion mobility spectrometry through the development of a Fourier transform mode of operation. Coupling Fourier transform ion mobility spectrometry with supercritical fluid chromatography as is proposed in this application will produce a powerful analytical instrument for both quantitative and qualitative determination of trace quantities of biologically important compounds that are not sufficiently volatile to be eluted by gas chromatography but are contained in mixtures too complex to be separated by liquid chromatography.